I can understand the argument of using different identities, but why not simply let the user decide and choose the number of Nsec they need? For instance, a public profile and its social graph are necessary when one wants to prove who one is in a DM context.

Seed phrase vs nsec is one debate, primary key vs alternative keys is another.

Seed phrases and nsecs have trade-offs. I'm not super opinionated about one vs the other. But the more formats keys have, the more confusing it is for users. The ideal UX should probably be "paste your secret" and the client can parse it as a hex private key, nsec, or seed words.

On primary keys vs alternative keys, I definitely agree that the ability to make single-purpose accounts is important. But the most common use case for chat is to start a conversation with someone you already know, which requires knowing their user id. Otherwise you lose the advantage that having a social identity layer gives you.

Maybe if you want to surface that decision, you could start the app in "secret mode", which encourages one-off identities, or "easy mode" which asks for their existing nostr account and allows for social discovery.

Speaking for myself, I would almost always use the insecure mode. Conversations partners knowing who I am is a feature, not a bug. If I ever do need to contact someone secretly, I should be able to bootstrap a fresh key from my normal social context without revealing my identity.